Catalytic reforming, or hydroforming, is a well-established industrial process employed by the petroleum industry for improving the octane quality of naphthas or straight run gasolines. In reforming, a multi-functional catalyst is employed which contains a metal hydrogenation-dehydrogenation (hydrogen transfer) component, or components, substantially atomically dispersed upon the surface of a porous, inorganic oxide support, notably alumina. Noble metal catalysts, notably of the platinum type, are currently employed, reforming being defined as the total effect of the molecular changes, or hydrocarbon reactions, produced by dehydrogenation of cyclohexanes and dehydroisomerization of alkylcyclopentanes to yield aromatics; dehydrogenation of paraffins to yield olefins; dehydrocyclization of paraffins and olefins to yield aromatics; isomerization of n-paraffins; isomerization of alkylcycloparaffins to yield cyclohexanes; isomerization of substituted aromatics; and hydrocracking of paraffins which produces gas, and inevitably coke, the latter being deposited on the catalyst.
In a reforming operation, one or a series of reactors, providing a series of reaction zones, are employed. Typically, a series of reactors are employed, e.g., three or four reactors, these constituting the heart of the reforming unit. Each reforming reactor is generally provided with a fixed bed, or beds, of the catalyst, typically a platinum catalyst or a metal-promoted platinum catalyst, which receive down-flow feed, and each is provided with a preheater or interstage heater, because the reactions which take place are endothermic. A naphtha feed, with hydrogen, or recycle hydrogen gas, is co-currently passed through a preheat furnace and reactor, and then in sequence through subsequent interstage heaters and reactors of the series. The product from the last reactor is separated into a liquid fraction, and a vaporous effluent. The former is recovered as a C.sub.5 + liquid product. The latter is a gas rich in hydrogen, and usually contains small amounts of normally gaseous hydrocarbons, from which hydrogen is separated and recycled to the process to minimize coke production.
The sum total of the reforming reactions occurs as a continuum between the first and last reactor of the series, i.e., as the feed enters and passes over the first fixed catalyst bed of the first reactor and exits from the last fixed catalyst bed of the last reactor of the series. During an on-oil run, the activity of the catalyst gradually declines due to the build-up of coke on the catalyst, and hence during operation, the temperature of the process is gradually raised to compensate for the activity loss caused by the coke deposition. Eventually, however, economics dictate the necessity of reactivating the catalyst. Consequently, in all processing of this type the catalyst must necessarily be periodically regenerated by burning off the coke at controlled conditions.
Two major types of reforming are generally practiced in the multi-reactor units, both of which necessitate periodic reactivation of the catalyst, the initial sequence of which requires regeneration, i.e., burning the coke from the catalyst. Reactivation of the catalyst is then completed in a sequence of steps wherein the agglomerated metal hydrogenation-dehydrogenation components are atomically redispersed. In the semi-regenerative process, a process of the first type, the entire unit is operated by gradually and progressively increasing the temperature to maintain the activity of the catalyst, decreased by the coke deposition, until finally the entire unit is shut down for regeneration, and reactivation, of the catalyst. In the second, or cyclic type of process, the reactors are individually isolated, or in effect swung out of line by various manifolding arrangements, motor operated valving and the like. The catalyst is regenerated to remove the coke deposits, and then reactivated while the other reactors of the series remain on-stream. A "swing reactor" temporarily replaces a reactor which is removed from the series for regeneration and reactivation of the catalyst, until it is put back in series. The cyclic method of regeneration offers advantages in that the catalyst, because it can be continuously regenerated, and reactivated, without shutting down the unit, suffers no loss in production. Moveover, because of this advantage the unit can be operated at higher severities to produce higher C.sub.5 + liquid volume yields of high octane gasoline than semi-regenerative reforming units.
Catalysts constituted of platinum and iridium, with or without the presence of an additional metal or metals, are known to be the most active of commercial reforming catalysts. The ability of iridium to promote platinum activity provides catalytic activities two to four times that of platinum and platinum-rhenium catalysts, depending upon the platinum and iridium loadings selected. A major liability of platinum-iridium catalysts is the ease in which iridium is agglomerated upon exposure to oxygen at high temperatures. This fact has restricted the wide application of platinum-iridium catalysts, especially to exclude their use in cyclic reforming units, since time-consuming regeneration procedures are required to avoid damaging the iridium. These regeneration methods utilize lengthy, low temperature coke burns in the presence of chloride to maintain the iridium in highly dispersed state. Low oxygen concentrations in the combustion gas are also employed during the combustion period to hold the flame front temperature below about 800.degree. F. (426.7.degree. C.). The use of chloride during this prolonged burning period also creates a number of troublesome problems. For example, the use of scrubbing equipment is required to remove the corrosive chloride-containing gases from the gas recycle stream. Moreover, volatile iron chlorides are formed by reaction of the chlorine with reactor walls, and the deposition of these iron salts on the reforming catalyst contributes to poor on-oil performance.
Platinum-iridium catalysts offer other benefits in addition to their high activity. The catalysts generate low levels of coke relative to other catalysts, e.g., platinum-rhenium catalysts which have enjoyed high commercial success, which has the effect of extending cycle length and directionally minimizing burn time where high temperatures can be employed. In contrast to platinum-rhenium catalysts, platinum-iridium and platinum-rhenium-iridium systems are more sulfur tolerant, and the use of feed sulfur at the lower levels which become possible lowers the level of hydrogenolysis of the feed to methane. This results in higher C.sub.5 + liquid volume yields of the product.
There thus exists a need, among others, for a novel reforming process employing a catalyst offering high activity, high sulfur tolerance, low coke formation, high C.sub.5 + liquid volume yield, and rapid catalyst regeneration. Platinum-iridium catalysts admirably satisfy the first three of these five enumerated requirements, but do not provide as high C.sub.5 + liquid volume yields as some other catalysts, e.g., platinum-rhenium catalysts, and require lengthy periods for regeneration of the catalyst, e.g., as contrasted with platinum and platinum-rhenium catalysts.